Thermal limit values of fuel loaded into the core of a boiling water nuclear power plant include two thermal limit values, that is, the thermal limit values for a maximum linear heat generation rate and a minimum critical power ratio. The linear heat generation rate represents an output per unit length of a fuel rod in a fuel assembly and the critical power ratio represents a ratio of a fuel assembly output (critical output), when boiling transition takes place, to an actual fuel assembly output. Both can be calculated with high accuracy by using the calculation result of a three-dimensional power distribution inside the core by a core monitoring system. Generally, the core monitoring system is activated by operators in a one-hour cycle or on an on-demand basis. The operators monitor whether or not the maximum linear heat generation rate and the minimum critical power ratio exceed the respective thermal limit values on the basis of the calculation result by the core monitoring system, judge whether or not the control rod operation and the core flow rate operation should be continued, and control the reactor output.
Recently, needs have been increasing for the use of an automatic power regulator system so as to save power involved in the reactor operation and for high-speed activation to improve economy. When the high-speed operation is executed by using the automatic power regulator system, an automatic thermal limit monitor for monitoring the thermal limit values in a short cycle is necessary as back-up means for the operators. An example of the conventional automatic thermal limit monitors is described in JP-A-51-67898. This reference describes a method of monitoring the thermal limit values by the steps of calculating an output distribution in a prescribed area inside a core by a one-dimensional neutron diffusion model, comparing and collating the one-dimensional neutron diffusion result with an indication value of a local power range monitor and executing a corrected one-dimensional calculation which adds a correction to the boundary condition. Whether or not the maximum linear heat generation rate and the minimum critical power ratio satisfy the respective thermal limit values is judged by such an automatic thermal limit monitor, and when they are judged as exceeding the thermal limit values, an automatic operation exclusion instruction is outputted to the automatic power regulator system. Incidentally, the term xe2x80x9cautomatic operation exclusionxe2x80x9d means the stop of the operation of the automatic power regulator system, and the automatic power regulator system must be activated once again to re-start the automatic operation.
According to the prior art technology described above, the maximum linear heat generation rate and the minimum critical power ratio can be calculated in a shorter cycle (about 10 seconds) than the calculation cycle (generally, one hour) of the core monitoring system for executing a three-dimensional power distribution calculation. To improve reliability, however, duplexing and further reduction of the cycle (to one second or below) are necessary in the monitoring operation of the thermal limit values by the automatic power regulator system. Because the processes for this purpose are complicated and trouble-some, duplexing and further reduction of the cycle have not yet been achieved by this prior art technology.
To further shorten the calculation cycle of the maximum linear heat generation rate and the minimum critical power ratio, therefore, it may be possible to employ an automatic thermal limit monitor which calculates the maximum linear heat generation rate and the minimum critical power ratio in the following way while the core monitoring system is executing its calculation. In other words, it may be possible to correct the maximum linear heat generation rate and the minimum critical power ratio as the calculation result of the core monitoring system by utilizing the values of the plant data (for example, the output of a local power range monitor) at the time when calculation is made by the core monitoring system and the plant data at the present moment and thus to briefly calculate the maximum linear heat generation rate and the minimum critical power ratio at the present moment.
When such an automatic thermal limit monitor is employed, duplexing for improving reliability can be achieved easily because the process is simple, and the cycle can be shortened, as well. Nonetheless, this method involves the problem that calculation accuracy of the maximum linear heat generation rate and the minimum critical power ratio is low. For this reason, the calculation error increases as the time passes from the calculation by the core monitoring system. In other words, calculation accuracy gradually gets deteriorated from immediately after the calculation by the core monitoring system, the calculation error reaches the maximum immediately before the next calculation by the core monitoring system and is again improved immediately after the next calculation by the core monitoring system.
Let""s assume hereby the case where the automatic thermal limit monitor is used to judge whether or not the maximum linear heat generation rate and the minimum critical power ratio satisfy the thermal limit values, and the automatic operation exclusion instruction is outputted to the automatic power regulator system when they exceed the thermal limit values. Because calculation accuracy of the brief calculation of the maximum linear heat generation rate and the minimum critical power ratio by the automatic thermal limit monitor is low as described above, the maximum linear heat generation rate and the minimum critical power ratio exceed the respective thermal limit values in many cases under such a core condition where the maximum linear heat generation rate and the minimum critical power ratio exist in the proximity of the thermal limit values. In order to briefly calculate the maximum linear heat generation rate and the minimum critical power ratio, the automatic thermal limit monitor is designed in such a fashion that the calculation result becomes rather conservative. Therefore, the possibility that the maximum linear heat generation rate and the minimum critical power ratio exceed the thermal limit values is high when the core condition is in the proximity of the thermal limit values, or when the control rod operation quantity is great during the period in which the core monitoring system executes the calculation.
The automatic power regulator system is stopped when the maximum linear heat generation rate and the minimum critical power ratio exceed the thermal limit values. However, because calculation accuracy of the maximum linear heat generation rate and the minimum critical power ratio by the automatic thermal limit monitor is low as described above, they are judged in many cases as exceeding the thermal limit values although they do not actually exceed the thermal limit values. In such cases, the automatic power regulator system must be repeatedly stopped and re-activated. Re-activation of the automatic power regulator system is very complicated and troublesome to practice because the links with other systems such as the control rod control system, the re-circulation flow control systems, etc., must be established once again. Consequently, it is difficult by the method described above to virtually use the automatic power regulator system.
It is an object of the present invention to provide a control system of a nuclear power plant, and a control method therefor, capable of easily executing automatic power regulation by an automatic power regulator system even when a maximum linear heat generation rate and a minimum critical power ratio are determined in a short cycle by a brief calculation.
To accomplish the object described above, the present invention provides a control system of a nuclear power plant which comprises a core monitoring system for determining a maximum linear heat generation rate and a minimum critical power ratio in a certain time interval; an automatic thermal limit monitor for determining a maximum linear heat generation rate and a minimum critical power ratio by utilizing the maximum linear heat generation rate and the minimum critical power ratio determined by the core monitoring system and plant data when the core monitoring system does not execute the calculation of the maximum linear heat generation rate and the minimum critical power ratio, comparing the maximum linear heat generation rate and the minimum critical power ratio so determined with thermal limit values set in advance for them, and outputting an operation hold instruction when at least one of the maximum linear heat generation rate and the minimum critical power ratio exceeds the thermal limit values; and an automatic power regulator system for outputting control signals to a re-circulation flow control system for controlling the re-circulation flow rate in a reactor and to a control rod control system for controlling the positions of control rods in the reactor, and holding the control signals when the operation hold instruction is outputted from the automatic thermal limit monitor.
The automatic power control system holds the control signals to the control rod control system and to the re-circulation flow control system when at least one of the maximum linear heat generation rate and the minimum critical power ratio determined by the automatic thermal limit monitor exceeds the thermal limit value. Therefore, when the actual maximum linear heat generation rate and the actual minimum critical power ratio are below the thermal limit values, the automatic power regulator system need not be re-activated, and the control of the control rods and the re-circulation flow rate can be started again smoothly. Even when the maximum linear heat generation rate and the minimum critical power ratio are determined in a short cycle by the brief calculation, therefore, automatic power regulation can be easily carried out by the automatic power regulator system. The present invention is effective for the continuation of the automatic operation particularly when the core condition exists in the proximity of the thermal limit values.
When the automatic power regulator system holds the control signal when it receives the operation hold instruction from the automatic thermal limit monitor, the present invention compares the maximum linear heat generation rate and the minimum critical power ratio determined by the core monitoring system with the thermal limit values, and stops the holding operation of the control signal by the automatic power regulator system when the maximum linear heat generation rate and the minimum critical power ratio are below the thermal limit values.
Because holding of the control signal by the automatic power regulator system is stopped when the maximum linear heat generation rate and the minimum critical power ratio that are determined highly accurately by the core monitoring system are below the thermal limit values, the automatic output regulating operation can be continued when the actual maximum linear heat generation rate and the actual minimum critical power ratio are below the set values.
When the automatic power regulator system holds the control signal as it receives the operation hold instruction from the automatic thermal limit monitor, the maximum linear heat generation rate and the minimum critical power ratio that are determined by the core monitoring system are compared with the thermal limit values, and when at least one of the maximum linear heat generation rate and the minimum critical power ratio exceeds the thermal limit values, the output of the control signal by the automatic power regulator system is stopped.
Because the output of the control signal by the automatic power regulator system is stopped when at least one of the maximum linear heat generation rate and the minimum critical power ratio that are determined highly accurately by the core monitoring system exceeds the thermal limit values, the automatic power regulating operation can be stopped when the actual maximum linear heat generation rate and the actual minimum critical power ratio exceed the thermal limit values.